linux-kernel - Re: [PATCH v5 03/13] iommufd: Add iommufd_verify_unfinalized

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Message-ID: <20241029185558.GZ6956@nvidia.com>
Date: Tue, 29 Oct 2024 15:55:58 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, will@...nel.org, joro@...tes.org,
	suravee.suthikulpanit@....com, robin.murphy@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com, shuah@...nel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, eric.auger@...hat.com,
	jean-philippe@...aro.org, mdf@...nel.org, mshavit@...gle.com,
	shameerali.kolothum.thodi@...wei.com, smostafa@...gle.com,
	yi.l.liu@...el.com, aik@....com, zhangfei.gao@...aro.org,
	patches@...ts.linux.dev
Subject: Re: [PATCH v5 03/13] iommufd: Add iommufd_verify_unfinalized_object

On Tue, Oct 29, 2024 at 09:18:05AM -0700, Nicolin Chen wrote:
> On Tue, Oct 29, 2024 at 11:49:07AM -0300, Jason Gunthorpe wrote:
> > On Fri, Oct 25, 2024 at 04:49:43PM -0700, Nicolin Chen wrote:
> > > To support driver-allocated vIOMMU objects, it's suggested to call the
> > > allocator helper in IOMMU dirvers. However, there is no guarantee that
> > > drivers will all use it and allocate objects properly.
> > > 
> > > Add a helper for iommufd core to verify if an unfinalized object is at
> > > least reserved in the ictx.
> > 
> > I don't think we need this..
> > 
> > iommufd_object_finalize() already does:
> > 
> > 	old = xa_store(&ictx->objects, obj->id, obj, GFP_KERNEL);
> > 	/* obj->id was returned from xa_alloc() so the xa_store() cannot fail */
> > 	WARN_ON(old);
> 
> It feels unsafe to carry on the iommufd_viommu_alloc_ioctl() until
> iommufd_object_finalize() as the function would touch the returned
> faulty viommu pointer? E.g. what if the viommu has an even smaller
> size than struct iommufd_viommu?

This is Linux just because the output came from a driver doesn't mean
we have to validate it somehow. It is reasonable to be helpful and
detect driver bugs, but if the driver is buggy it is still OK to
crash.

So you don't *have* to check any of this, if the driver didn't use the
right function to allocate the memory then it will go bad pretty fast.

Improving the xa_store() is something that will detect more kinds of
bugs everywhere, so seems more worthwhile

> I think we'd need the same change in iommufd_object_abort() too.

Makes sense

Jason